"My friend was sexually assaulted"
I love Sylvia Boorstein's book title: "Don't just do something, sit there." It's the best advice for someone supporting a sexual violence survivor.
If they start to cry, to rage, to shake--let them.
Don't pat them and say, "there, there." Don't tell them, "it's okay." Don't try to calm them down. By not "comforting" them, you are giving them an incredible gift: the safety to express their emotions.
Don't touch them without permission. For someone who has been violated, it can feel like another attack. (A hug with permission may be okay.)
Don't tell them what to do. They had their choice taken away. Give it back to them.
Don't tell your own story, demanding their attention. Right now, they need yours.
Don't press for details. Don't ask questions. Don't seek clarification.